For obtaining the lowest possible toxic emissions, it is desirable to detect and replace catalytic converters having a reduced conversion performance. Statutory requirements exist for systems which permit detecting the reduction of conversion performance with on-board means (on-board diagnosis). In known systems, such as disclosed in U.S. Pat. No.4,622,809, the signals of a first lambda probe arranged forward of the catalytic converter and the signals of a second lambda probe arranged rearward of the catalytic converter are compared for diagnostic purposes. The first lambda probe is also the control probe. U.S. patent application Ser. No. 738,360, filed Jul. 31, 1991, now U.S. Pat. No. 5,203,165, also shows a system of the kind described above.
For the above purpose, the periodic oscillation of the lambda value is used with the periodic oscillation being caused by the two-level characteristic of the mixture control method. This periodic oscillation occurs at least in steady-state operating conditions. The mean value of this oscillation corresponds ideally to the control desired value such as lambda=1. The half periods, in which a lean mixture is present, are associated with an oxygen excess in the exhaust gas; whereas, an oxygen deficiency is present in the other half periods because of a rich mixture composition. Based on its capability to buffer the oxygen deficiency, as well as the oxygen excess within certain limits, the catalytic converter exercises an averaging influence on the oxygen content of the exhaust gas. This influence is detected in known methods by a comparison of the amplitudes of the signals of the exhaust-gas probes arranged forward and rearward of the catalytic converter and is applied for evaluating the conversion performance.
A disadvantage of the above-described method is seen in the very definite dependency of the signal amplitudes of other individual parameters such as the temperature and the chemical and thermal aging of each individual one of the two probes. The evaluation of the conversion performance according to these methods is therefore burdened with corresponding uncertainties. On the one hand, this leads to the situation that catalytic converters which no longer satisfy the statutory requirements cannot be detected so that the toxic emissions increase. On the other hand, it is also possible that catalytic converters which are still adequate are already evaluated as being defective and, possibly, are exchanged too early with economic disadvantages for the owner of the motor vehicle.